This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In contrast to pathogenic lentiviral infections, chronic SIV infection in its natural host is characterized by a lack of chronic immune activation. In recent studies we have shown that acute events following SIV infection in rhesus macaques differ from those occurring in the natural host sooty mangabeys with regards to the presence of CD4+ T lymphocyte apoptosis and induction of TNF-receptor associated apoptosis-inducing ligand. The goal of this project is to better define differences in the early host response to SIV in pathogenic and nonpathogenic SIV infection.